1. Field of the Invention
The present invention relates to a driver for a diode laser, and more prticularly, to a driver which is not affected by optical feedback.
2. Description of the Prior Art
Diode lasers are being used increasingly as light sources in applications such as laser printers. The use of diode lasers reduces the cost of the printer and permits a drastic reduction in the size and complexity of printer. Diode lasers can be modulated directly by simply modulating the drive current, and thus, no external modulators are required. Although these advantages provide substantial benefits, there is a problem in using diode lasers in apparatus which produce optical feedback.
Optical feedback occurs when a minute fraction of the laser output is reflected back into the diode laser by an external optical surface in the apparatus. The effects of optical feedback on diode lasers have been noted in a paper entitled "Low-frequency intensity fluctuation in laser diodes with external optical feedback, " App. Phys. Lett. 38(4), Feb. 1981. Noise and instability caused by optical feedback can be especially troublesome in analog applications such as coninuous-tone scan printers. As a result of the optical feedback, there is a nonlinear relationship between drive current and diode laser output, and this nonlinearity makes calibration very difficult in a continuous-tone scan printer. One common technique used to prevent optical feedback is to employ an optical isolator which blocks the backward propagation of light into the diode laser cavity. The optical isolator could include, for example, a linear polarizer and a quarter-wave plate. An optical isolator of this type, however, produces circular polarization, and thus, it is not suitable for certain applications, for example, applications which use dichroic beam combiners. Other known types of optical isolators are bulky and expensive.
In U.S. Pat. No. 4,799,069, there is disclosed a laser printer comprising a diode laser which is modulated in accordance with levels of an analog signal. In order to prevent mode hopping noise and optical feedback induced intensity noise, the output of the diode laser is turned off or reduced in level for a certain amount of time in each time period corresponding to a pixel of the printed image. One problem with the apparatus disclosed in the patent is that complex circuitry, including a digital-to-analog converter, is needed to control the current in the laser diode. A further problem is that the image signal is pulse amplitude modulated at a frequency which is too low to overcome the effects of optical feedback.